Shifting Icebergs May Have Forced Penguin Evolution

The breakup of
giant icebergs may have forced minor evolutionary changes in penguins over the
past 6,000 years, a new study suggests.

The Antarctic
iceberg chunks, which break off the continent now and then, are thought to have
blocked the swim paths of Adelie penguins returning home to their colonies.
Some of the penguins were forced to become immigrants in other colonies, where
they established new homes and interbred with the locals.

As a result,
genetic changes that might otherwise have remained isolated became widespread
among the different colonies. The result is what scientist call microevolution.

Other
examples

Microevolution
involves small-scale genetic changes in a species over time. The classic example
is a color change undergone by British pepper moths in response to changing
levels of air pollution. The acquisition of antibiotic resistance by bacteria
and the trend towards tusk-less elephants in Africa are also examples of
microevolution at work.

Because it is
so well documented, even people who don't believe that evolution can lead to the
creation of new species accept that microevolution occurs.

Most
microevolution studies involve change over very short time periods, on the
order of decades or a few hundred years. The detection of microevolutionary
changes over longer time periods has been difficult because it requires that
ancient DNA deposits be found together with samples from modern populations of
the same species.

Adelie
penguins may be the ideal candidates for such research. The penguins often
live, breed and die in the same colonies where they were born and where their
ancestors before them lived. And the remains of ancestor birds are well
preserved in distinct layers of the frigid terrain, making fossil dating relatively
easy.

For the study,
the researchers extracted DNA from the bones of 6,000-year-old penguins and
compared them to the DNA of their modern descendents. In particular, they
looked at various genes made up of short stretches of repeat DNA sequences,
called "microsatellite DNA."

Comparisons
between the ancient and modern sequences revealed that the DNA sequences for
some of the genes had gotten longer over time. The frequency of some of the
different genes had changed as well: New variations had come into existence while
others had been phased out.

Iceberg-driven
evolution

One surprising
finding was that there wasn't much genetic variation between different penguin
colonies. Because adult penguins tend to return to the nesting colonies used by
their ancestors, little interbreeding would be expected to occur between
colonies. Over time, this would lead to genetic differences between colonies.

"Originally
these colonies were almost certainly very different from each other," said
David Lambert from Massey University in New Zealand, the principal investigator
of the study. "[But] over time they've become the same."

Something was
causing the penguins to break from their normal behavior and interbreed with
members of different colonies. One idea was that the breakup of mega-icebergs was blocking the swim paths of
penguins and forcing them to migrate to more accessible colonies.

To test their
hypothesis, the researchers tracked the movements of nearly 10,000 penguins.
The birds had been banded as chicks and came from three different colonies. In
2001, a section of the mega-iceberg B-15A grounded near the colonies. By tracking the banded
penguins, the researchers discovered that many of them couldn't return to their
home colonies. Some of the displaced birds were able to assimilate into other
colonies and to interbreed with the locals.

The
researchers think icebergs may have been a constant evolutionary pressure for
the penguins and estimate that there have been about 200 such events within the
past 10,000 years.

The study was
detailed in the Nov. 7 issue of the journal for the Proceedings of the
National Academy of Sciences.

Ker Than

Ker was a staff reporter for Live Science and Space.com and has freelanced for various outlets, including New Scientist and Popular Science. He has degrees from the University of California, Irvine and New York University, including a masters degree in science journalism.